Quasi-Talbot effect with vortex beams and formation of vortex beamlet arrays

Beams with an orbital angular momentum, or vortex beams, attract growing interest. Here we report results of simulations and experiments at the Novosibirsk Free Electron Laser (NovoFEL), in which Bessel vortex beams with topological charges of ±1 and ±2 and a wavelength of 130 μm diffracted on two-dimensional periodic gratings. In our case self-images of the holes of the grating were not observed and obviously could not be observed. Nevertheless, in the planes corresponding to the main and fractional Talbot planes we observed complex periodic patterns depending on the grating period, the size of the holes, and the diameter of the incident beam. Conditions were found under which arrays of annular vortex beamlets with topological charges corresponding to the charge of the beam illuminating the grating, but with charge-independent diameters, are formed at some Talbot distances.